In commonly owned U.S. Pat. No. 4,081,611 there has been disclosed a PCM station of this type, acting as a coupling network or transit exchange between incoming and outgoing links, in which arriving messages are fed to a line unit via a receiving interface while departing messages pass through a transmitting interface. Between these two interfaces the line unit includes, inter alia, a malfunction detector which is controlled by a timing circuit and can send alarm signals to an insertion circuit enabling their interpolation with outgoing message signals. The station dialogues with a central processor which supervises its operation and sends instructions concerning the connections to be established.
In conformity with international regulations, data transmitted in such a PCM/TDM telecommunication system are organized in a succession of frames each consisting of a multiplicity of channels accommodating respective multibit words, usually 8-bit bytes; a channel may be defined as a time slot subdivided into n clock cycles or phases. For proper synchronization of clocks controlling the operations at the transmitting and at the receiving end of a signal path, certain alignment words are used in an initial channel of each frame (referred to hereinafter as the No. 0 channel). These alignment words generally differ from each other in odd-numbered or "first" and even-numbered or "second" frames; thus, they will alternately assume two different forms referred to hereinafter as "word A" and "word B". Word A, appearing in the No. 0 channel of the recurrent first frame, may have a large number of its bits (e.g. 7 out of 8) arranged in an invariable configuration; word B, present in the No. 0 channel of the immediately following second frame, need only have one particular bit in a predetermined time position.
When data to be sent out from such a station (e.g. in response to instructions from the processor) have not yet been organized into frames, bytes assigned to the No. 0 channel will have to be given bit configurations conforming to these alignment words. Bits not included in the invariable configurations can still be used, however, to carry information to the remote terminal.